Why have the police now declared that the McCanns are suspects?

The police think that Madeleine is dead, although they have not revealed why. They told Kate and Gerry that forensic tests had found “bodily fluids”, thought to be from Madeleine, in a Renault Scenic that the couple had hired 25 days after her disappearance.

The police also told the McCanns that sniffer dogs trained to detect the smell of dead bodies had shown unusual interest in the Renault and had reacted to her jeans, T-shirt and Madeleine’s Cuddle Cat toy.

The police suggest Kate McCann accidentally killed her daughter and disposed of the body later.

How strong is the forensic evidence of “bodily fluids”?

It is not clear whether any blood was found during forensic tests on samples from the car or the McCanns’ clothing, or whether other material, such as DNA, was involved. Forensic experts say that there are several reasons for caution. First, in such a long-running investigation there is a risk of crime scenes becoming contaminated; there is also a risk of cross-contamination during the testing of different samples.

Second, it would not be surprising for traces of Madeleine’s DNA to appear in the car or on clothes after she disappeared. Cells containing her DNA could easily be transferred from, say, her cuddly toy that Kate has carried with her everywhere.

Yesterday Jon Corner, a godfather of one of the McCanns’ children, said: “The Cuddle Cat was reeking with Madeleine’s DNA. That easily explains why DNA has been found in the hire car and on clothing that Kate bought after Madeleine disappeared.”

How was the car tested?

In an unsatisfactory manner, it appears. The silver Renault was examined by the police on August 7 and samples were sent for analysis in Britain. The car was handed back to the McCanns two days later. The couple have been using it ever since.

Dr Hugh White, a Home Office pathologist based in Bristol, said he could not understand why the car was held by the police for only two days: “In this country the car would have been stripped down and the forensics team would be crawling all over it. There is no way it would have been handed back once blood had been found in it. It would be kept as evidence and used as an exhibit if there was a court case.”

Could a sniffer dog have detected the scent of a corpse in the car and on clothing belonging to Kate?

Dogs are trained, using body parts from mortuaries, to react to the distinctive smell of human remains.

British experts say this smell can persist in a car, which is difficult to clean, for weeks. However, they say that once clothes are washed it would be hard for dogs to detect a meaningful smell.

One forensic expert said: “You can imagine the fun that a defence lawyer would have with a dog that smelt a corpse. If a dog smelt a corpse, well, whose corpse?”

Might the McCanns have accidentally given Madeleine an overdose of sedative?

British doctors believe it is extremely unlikely for parents - especially doctors such as the McCanns – to give a child an overdose of regular medicines intended to sedate children.

Antiallergy medication, such as Piriton, can be given as a sedative but at least a bottle would be needed to cause an overdose. Professor Saad Shakir, of the Drug Safety Research Unit in Southampton, said: “It is very difficult to persuade a child to take that amount.”

The McCanns have said they only gave their children Calpol, which is a painkiller containing paracetamol. It does not have a sedative effect. If administered in huge doses, such as a couple of bottles, it would damage the liver - but the effect would take several days to become apparent. Nor is a child who has overdosed doomed: they can be given an antidote to neutralise the effect of the drug.

Could the McCanns still be fooling everyone?

If they were, it would be a truly extraordinary effort. “It is very difficult for two people to lie over a death, whether it was accidental or deliberate,” said Mike Berry, senior lecturer in forensic psychology at Manchester Metropolitan University. “I cannot see how they would be able to keep up a lie for so long under this media attention.”

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